Various processes have been proposed in the prior art for boronation of aromatic hydrocarbons. For example, processes are known for lithionation, halogenation or boronation after converting to a trifurate of a benzene ring, examples of which include (1) a process using aryl halide or aryl trifurate and pinacol diboron (P. Rocca et al., J. Org. Chem., 58, 7832, 1993), (2) a process involving reaction with boric ester following lithionation of an aromatic ring, and (3) a process involving reaction with boric ester following reaction of aryl halide with magnesium (A. R. Martin, Y. Yang, Acta. Chem. Scand., 47, 221, 1993).
In addition, known examples of direct boronation of benzene include (4) a process that uses a boron halide (T. R. Kelly et al., Tetrahedron Lett., 35, 7621 (1994), P. D. Hobbs et al., J. Chem. Soc. Chem. Commun., 923 (1996), T. R. Hoye, M. Chen, J. Org. Chem., 61, 7940 (1996)), (5) a process that uses an Ir-based catalyst (Iverson, C. N., Smith, M. R., III. J. Am. Chem. Soc., 121, 7696 (1999)), (6) a process that uses an Re-based catalyst (Chen. H., Hartwig, J. F., Agnew. Chem. Int. Ed., 38, 3391 (1999)), (7) a process that uses an Rh-based catalyst (Chen, H., Hartwig, J. F., Science, 287, 1995 (2000), Cho, J. Y., Iverson, C. N., Smith, M. R., III. J. Am. Chem. Soc., 122, 12868 (2000), Tse, M. K., Cho, J. Y., Smith, M. R., III. Org. Lett., 3, 2831 (2001), Shimada, S., Batsanov, A. S., Howard, J. A. K, Marder, T. B., Angew. Chem. Int. Ed., 40, 2168 (2001)), and (8) a process that uses an Ir-based catalyst (Cho, J. Y., Tse, M. K., Holmes, Science, 295, 305 (2002), Ishiyama, T., Takagi, J., Ishida, K., Miyaura, N., Anastasi, N. R., Hartwig, J. F., J. Am. Chem. Soc., 124, 390 (2002)).
However, there are few examples of boronation reactions of aromatic heterocyclic compounds, with the only known example being (9) a process in which silver acetate is allowed to act on indole followed by reaction with borane followed additionally by hydrolysis (K. Kamiyama, T. Watanabe, M. Uemura, J. Org. Chem., 61, 1375 (1996)).
Although the processes (1) through (9) are known as examples of boronation of an aromatic ring as mentioned above, these examples of the prior art have the following disadvantages. The processes of (1) through (3) have a large number of steps for carrying out lithionation, halogenation or trifluorination of a benzene ring, thereby resulting in problems with industrial production. Moreover, process (1) only uses one of the two borons of the diboron used, thereby making it uneconomical, while processes (2) and (3) are subjected to considerable restrictions on the functional groups of the substrate used due to going through a highly reactive intermediate. Process (4) has the disadvantages of harsh reaction conditions, low yield and the formation of isomerism in the case of substrates having functional groups. In the processes of (5) through (7), the catalyst is difficult to acquire while also having the problem of requiring harsh reaction conditions. In process (8), although there are some processes that enable boronation of a benzene ring to take place with high yield and in a single step, there are no known application examples for the aromatic heterocyclic ring. In the process of (9) involving application of boronation to a complex ring, there is the disadvantage of having to react borane, which is associated with the risk of toxicity and explosiveness, after allowing harmful silver acetate to act on indole. In consideration of these circumstances, there is a need for the development of a novel boronation reaction for aromatic complex rings that is able to overcome the aforementioned problems.